There are different types of bumblebee gobies, but all thrive in brackish water. Some can "survive" in
FW, but their lifespan will be much shorter and they will be more likely to suffer disease and it's just generally not healthy or comfortable for them.
Pet store owners and attendants don't "flinch" so to speak because they are concerned with profit and sales, not the quality of life of the fish.
Add a tablespoon of marine salt per gallon of water for brackish water. That's all there is to it, but it will make a world of difference for his health. He will do really well in a 5g brackish to himself (because your FW fish can't have the salt).
Best wishes to your fish!
Oh, here's a nice site to read about your new goby

:
http://aqualandpetsplus.com/Oddball,...bee%20Goby.htm